The domain registrar you bought the domain from can help you with this. They should either be able to do it for you or let you know how to do it. Just ask them to “forward” or “redirect” the second domain to your main domain.

Do not change the nameservers of your second domain to our nameservers. This will not create a 'forward/redirect'!

Absolutely! You can have as many domains as you'd like forward to your OPP site. Just contact the registrars for the domains you would like to forward – they should be able to tell you how to do this. Just ask them to “forward” the domain(s) to your main OPP website domain.

Only one domain can be directly associated with your OPP website, and OtherPeoplesPixels can only manage/own one domain per account. However, you can have as many domain 'forwards' to your OPP domain as you wish.

Just purchase the additional domains you wish to own, and set them to 'forward' or 'redirect' to your OPP domain.